Power over Ethernet is a system which allows electrical power to be passed down Ethernet cabling along with data. This permits a single Ethernet cable, such as category 5 or category 6 cabling, to provide both a data connection and electrical power to devices; such as wireless access points, IP cameras, and IP telephones, without the need to run separate power and data cabling. This can save considerable cost when installing a new system or when changes need to be made to an existing system, because it eliminates the need for an electrician to install a local AC power point when the system is installed or move the AC power point, when a device on the network is moved and is no longer close to a power outlet.
These systems are often subject to IEEE standards, such as IEEE standard 802.3 or the current version, IEEE Std 802.3-2012. The higher power amounts available in newer systems has led to their being utilized for backup power supplies, which must always be functional, and LED lighting systems, where some vital functionality must be maintained even though the light is simply turned off. The IEEE standard requires that the power to the PD from the PSE be removed if the current consumption falls below a predetermined limit, such as 5 mA or 10 mA. In order to maintain power to the PD, the PD may provide a maintain power signature (MPS) which is an electrical signature assuring the PSE that the PD still requires power. A valid MPS consists of a minimum DC current, such as 10 mA or a 10 mA pulse at least 75 ms in duration delivered at least once every 325 ms, and an AC impedance lower than 26.3 KΩ in parallel with 0.05 μF. In addition, the new IEEE standard 802.3bt will most likely use the same technique but with different current level and timing values. In addition, 802.3bt will likely also remove the AC impedance requirements.
Current systems either maintain the power drawn by the PD above the minimum or require an activation signal from an external source in order to provide current pulses to meet the MPS requirements.
Accordingly, there is a need for a circuit within the PD that automatically determines the need for MPS signal, without the need for an externally generated signal, and which provides the MPS pulses in order to maintain power to the PD while minimizing the power consumed.